Command substitution

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In computing, command substitution is a facility originally introduced in the Unix shells that allows a command to be run and its output to be pasted back on the command line as arguments to another command. Shells typically do this by creating a child process to run the first command with its standard output piped back to the shell, which reads that output, parsing it into words separated by whitespace. Because the shell can't know it has all the output from the child until the pipe closes or the child dies, it waits until then before it starts another child process to run the second command.

Syntax and semantics[edit]

This C shell example shows how one might search for all the C files containing the string malloc using fgrep and then edit any that are found using the vi editor. The syntactical notation shown here, ` ... `, using backquotes as delimiters, is the original style and is supported by all the common Unix shells.

#!/bin/csh
vi `fgrep -l malloc *.c`

Objections have been raised to both the syntax, how it's typed, and the semantics, how it works.

The syntax has been criticized as easy to type, an important factor for an interactive command processor, but awkward to nest, putting one command substitution inside another, because both the left and the right delimiters are the same.[1] Bash 2.0[2] and the Korn shell (ksh)[3] solved this with an alternative notation, $( ... ), borrowing from the notational style used for variable substitution:

#!/bin/bash
vi $(fgrep -l malloc *.c)

The semantics, breaking the output into words at whitespace, has also been criticized. It worked well on early Unix syntax where filenames never contained spaces but it doesn't work at all well on modern Windows and Linux systems where filenames certainly can contain spaces.[4] In either of these previous examples, if any of the filenames matched by the *.c wildcard contains a space, that filename will be broken into two separate arguments to vi, clearly not what was intended. Hamilton C shell solved this with a new double backquote notation, `` ... ``, that parses into words only at line breaks.[5]

History[edit]

Command substitution first appeared in the Bourne shell,[6] introduced with Unix 7th Edition, released in 1979, and has remained a characteristic of all later Unix shells. The feature has since been adopted in the programming languages Mythryl, Perl, PHP, and Ruby. It appears in Microsoft's CMD.EXE and Powershell under Windows. In the former it is called a subexpression and uses the $(...) format and in the later it is restricted only to the FOR command.

Expression substitution[edit]

A related facility, expression substitution, is found in the languages Common Lisp and Scheme, invoked by using the comma-at operator in an expression marked with the backquote (or "quasiquote") operator, and in ABC, by using an expression enclosed between backquotes inside a text display (string literal). For example, the ABC command WRITE '2 + 2 = `2+2`' produces the output 2 + 2 = 4.

In a sense this is the inverse of the eval function (in languages that have one): expression substitution turns an expression into a string, and eval turns a string into an expression.

See also[edit]

References[edit]

  1. ^ Unix Power Tools: 45.31 Nested Command Substitution.
  2. ^ Bash Prompt HOWTO: 3.3. Command Substitution.
  3. ^ Rosenblatt, Bill; Arnold Robbins (2002). Learning the Korn Shell (2 ed.). O'Reilly Media, Inc. p. 127. ISBN 978-0-596-00195-7. Retrieved 2010-07-20. The syntax of command substitution is: $(Unix command) The command inside the parenthesis is run, and anything the command writes to standard output (and to standard error) is returned as the value of the expression. 
  4. ^ Johnson, Chris (2009), "8", Pro Bash Programming: Scripting the Linux Shell, New York, NY: Springer-Verlag New York, Inc., p. 84, retrieved December 19, 2014, File names containing spaces are an abomination, but they are so common nowadays that scripts must take their possibility (or should I say inevitability?) into account. ... The result of command substitution is subject to word splitting 
  5. ^ Hamilton C shell User guide: I/O redirection: Command substitution, Hamilton Laboratories, retrieved December 19, 2014 
  6. ^ Dahdah, Howard. The A-Z of Programming Languages: Bourne shell, or sh, An in-depth interview with Steve Bourne, creator of the Bourne shell, or sh, Computerworld, March 5, 2009.